Rey knows how to fly because shes been around ships all her life, she knows the falcon because Unkar Putt owns it and shes probably been in it many times. In Star Wars, knowing how to fly is basically knowing how to drive.
She knows the Falcon inside out, yet claims that it’s ‘garbage’ before later teaching Han about compressors inhibiting the hyperdrive and auxiliary fuel pumps etc etc. It’s odd that someone with such intimate mechanical knowledge of the galaxy’s most famous hot rod would be so disparaging of it before boarding.
It hasn’t flown in years and it has the compressor that Rey doesn’t seem to want to deal with. I think that is why she called it garbage. She wanted the small, fast ship (which she may have flown before) over the old, unmoving hulk with questionable engineering.